1. Field of the Invention
The present invention relates to an electrophotographic machine, and, more particularly, to a method and apparatus for determining a position of a laser beam in an electrophotographic machine, such as a laser printer.
2. Description of the Related Art
In an in-line color laser image printing process, the print medium typically passes through four color developing stations in series, with the colors being black, magenta, cyan and yellow. In order for the multi-color laser printer to print at the same speed as a monochrome laser printer, photoconductive drum exposures must occur for all four colors simultaneously. Thus, alignment of the four color developing stations in both the process direction (feed direction of the print medium) and scan direction (across the page) is critical.
The process location of each scanning laser beam must overlap to prevent color misregistration in the process direction. Each color must have an adjustment to correct for process direction misalignment because each color has a scanning laser beam following a separate optical path. Although the laser beams can be aligned when the laser printer is first assembled, thermal changes occurring during operation of the laser printer can cause subsequent misalignment of the laser beams.
It is known to use a horizontal synchronization (HSYNC) sensor to determine the location of the start of the scan of the laser beam across the photoconductive drum. The HSYNC sensor has a rectangular photosensitive surface which is placed somewhere in the laser print head near the start of a scan line. When the laser beam strikes the HSYNC sensor surface at the start of a scan line, the photodiode sensor detects the presence of the laser beam and thereby identifies, the location of the laser beam. Such rectangular sensors, however, cannot be used to determine a location of the laser beam in the process direction.
What is needed in the art is a low-cost method of aligning multiple laser beams in a process direction of a laser printer such that all of the laser beams can operate simultaneously to thereby achieve the same printing speed as that of a monochrome laser printer.
The present invention provides a method of real time detection of process direction location of multiple scan lines of a multicolor electrophotographic machine, such as a laser printer.
In one embodiment of the method of the present invention, an aperture over a standard photodiode is used to generate timing pulses proportional to the process position. In this case, a lens only functions to collect the light onto the photodiode detector, not to maintain process direction information fidelity. In another embodiment, the aperture or mask is applied directly to the photodiode, a subassembly adjustment is used, and the lens is eliminated altogether. In addition, a subassembly adjustment can be used with a position sensitive device (PSD) sensor without a lens.
The invention comprises, in one form thereof, a laser beam sensor assembly in an electrophotographic machine. A sensor device detects a position of a laser beam in a process direction. The sensor device has a detection range in the process direction. The sensor device has at least one slot or at least one projection. A fixed structure has an other of the at least one slot and the at least one projection. The at least one slot and the at least one projection coact to allow the sensor device to slide in the process direction to a location such that the laser beam is at a desired position within the detection range of the sensor device.
The invention comprises, in another form thereof, a method of detecting a change in a position of a laser beam in a process direction in an electrophotographic machine. The method includes providing a sensor to detect the position of the laser beam in the process direction. The sensor has a detection range in the process direction. The position of the laser beam in the process direction relative to the detection range of the sensor is detected. The sensor is moved in the process direction until the laser beam is at a desired position within the detection range of the sensor. The position of the laser beam in the process direction is monitored. The monitoring is performed using the sensor.
An advantage of the present invention is that laser beam drift in the process direction can be detected.